See paragraph 2 here:
http://en.wikipedia.org/wiki/Unlawful_combatant
It must be remembered that 'the usual rules of engagement' didn't/don't apply to many forms of conflict. For example, during the WWII, Japan was not a signatory to the Geneva Convention, the Viet Cong didn't subscribe to such conventions and both sides in Syria certainly don't.
Even when nations sign up to conventions upon the treatment of their enemies, they frequently try to circumvent them, For example (until Obama reversed the policies) the USA got around the UN ban on the use of torture by simply redefining 'torture' to exclude both waterboarding and sleep deprivation (which just happened to be the USA's preferred methods of torture).